The characteristics of the synapse between postganglionic sympathetic nerves and the smooth muscle cells of small pulmonary arteries of dogs will be determined under normoxic conditions. Subsequently, studies will be done to determine whether synaptic events in these vessels change under the influence of acute or chronic hypoxia. Specifically it is planned to examine whether under hypoxic conditions, the synthesis, retention, release, reuptake and enzymatic degradation of the adrenergic neurotransmitter norepinephrine (NE) are altered. The impact on the pulmonary artery of known modulators of NE function (activation of presynaptic alpha and Beta receptors, serotonin, angiotensin II and histamine) will be examined during low and high oxygen tensions. The technics to be used include superfusion of isolated pulmonary arterial strips labeled with (3H)NE; these will be studied under basal conditions and during electrical stimulation. In addition isolated perfused pulmonary arteries will be used to examine the possible involvement of lung parenchyma in the enhancement of hypoxic vasoconstriction.